Research on the low-temperature degradation of dental zirconia ceramics fabricated by stereolithography.

Stereolithography is a promising method of fabricating zirconia ceramics with high strength and accuracy. However, studies of the aging effects on zirconia ceramics fabricated by this technique are lacking. The purpose of this in vitro study was to evaluate the aging effects on the crystalline content, microstructure, and mechanical properties of yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) printed by stereolithography apparatus (SLA) and digital light processing (DLP) compared with those of zirconia milled by computer numerical control (CNC). Bar-shaped specimens were fabricated after layer-by-layer printing, debinding, and sintering by SLA and DLP. Specimens milled and sintered by CNC were used as controls (n=24/material). The specimens were divided into 12 groups (n=6) and aged (0/5/10/15 hours, 134 °C, 0.2 MPa), after which the crystalline content, microstructure, and mechanical properties were evaluated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and 3-point bend tests. The flexural strength and monoclinic (m) phase content were statistically evaluated (α=.05). The XRD results showed that an m peak was not detected for any of the tested materials before aging. The m-phase content was the highest for SLA (5/10/15 hours: 19.64%/34.76%/41.88%), followed by DLP (5/10/15 hours: 9.62%/21.76%/28.43%) and CNC (5/10/15 hours: 2.29%/7.77%/7.66%). The SEM images showed zirconia grain fragments for DLP and grain pullout for SLA, while surface defects were not obvious for CNC. Within the materials, the flexural strength was the highest for SLA after aging for 5 hours (1010.3 MPa), followed by 10 hours (913.06 MPa) and 15 hours and 0 hours, which exhibited no difference (0/15 hours: 776.71/814.28 MPa) (P<.001). The flexural strength for CNC and DLP did not significantly change after aging for 5 hours, 10 hours, and 15 hours (P>.05). The flexural strength for CNC was always more than 1200 MPa, and that for DLP was approximately 800 MPa before and after aging. Although the m-phase content for SLA and DLP increased with the aging time, the mechanical properties did not significantly decrease, indicating the stability of both materials.

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